1. Technical Field
This disclosure relates to an apparatus and method for controlling a modulation signal for modulating a phase locked loop.
2. Description of the Related Art
Phase locked loops have a variety of applications, for example in frequency generation in wireless communication equipment. A basic phase locked loop comprises a variable frequency oscillator whose frequency and phase is controlled by an error signal, the error signal being generated by a phase detector that compares the phase of the oscillator output signal with the phase of a reference signal. The loop contains a low pass filter for filtering the error signal before it is applied to control the oscillator. Typically the oscillator operates at a higher frequency than the reference signal, and the loop includes a divider for dividing the oscillator output signal, such that the phase detector compares the divided signal with the reference signal.
The loop performance depends, among other things, on the gain of the oscillator, the gain being a measure of the change of oscillator frequency in response to a change in the error signal. As the oscillator is typically an analogue circuit, the loop performance is susceptible to variations in analogue circuit characteristics, due to, for example, temperature or voltage changes or manufacturing process spread, which lead to variations in oscillator gain.
In a wireless transmitter, in which a carrier signal is modulated by a modulation signal, it can be advantageous to apply the modulation to a phase locked loop, rather than to a mixer which would entail an additional RF circuit. A modulation signal can be applied to the input of the oscillator and/or used to control the division ratio of the divider. Schemes in which the modulation signal is applied to both the oscillator and the divider are known as two-point modulation schemes. However, variations in oscillator gain will lead to variations in the modulated carrier signal. In particular, if the oscillator gain is ideal, the transfer function for the modulation from the input of the oscillator to the output of the phase locked loop, that is the output of the oscillator, is an all-pass filter. However, if the oscillator gain is too high, high frequencies of the modulation signal are amplified too much, and if the oscillator gain is too low, high frequencies of the modulation signal are amplified too little.
Therefore, it is desirable to provide a way of calibrating a phase locked loop to compensate for variations in oscillator gain. Such calibration can be performed by scaling the modulation signal applied to the input of the oscillator. Successful calibration requires appropriate control of the degree of scaling.
WO 2004/034564 discloses a scheme for calibrating oscillator gain in which a correction signal is derived from a phase error signal, and in which the polarity of the error signal is measured by correlating the error signal with a modulation signal.
US 2007/0103240 discloses a scheme for gain calibration of a digitally controlled oscillator in which a phase error signal is sampled, either continuously or when zero crossings occur in an input data frequency control word, a gradient is generated from the samples and by using the sign of the data, and the oscillator gain estimate is adjusted in accordance with the gradient.